Add custom entrypoint mapping env vars to cli args

[nathan-az]

Cleaned up dockerfile, and parses environment variables with the SM_VLLM_ (sagemaker vllm) prefix to environment variables.

This is due to the fact that - as far as I know - vLLM does not support passing engine args as environment variables. TGI's flexibility is through using clap which allows args to be set either way (src).

Key changes:

• New file - entrypoint for sagemaker with mapping, invoking same api server module
• Slightly restructuring dockerfile to clean up requirements. Adds intermediate vllm-openai-base step with the additional pip requirements, which will be required for all openai-style servers. Derives from this for both the vllm-sagemaker image and vllm-base image